1. Field of the Invention
The present invention relates to microelectromechanical systems (MEMS) and, in particular, relates to the fabrication of MEMS components having a protective wafer level cap.
2. Discussion of the Related Art
Microelectromechanical system (MEMS) components are being progressively introduced into many electronic circuit applications and a variety of micro-sensor applications. Examples of MEMS components are electromechanical motors, radio frequency (RF) switches, high Q capacitors, pressure transducers and accelerometers. In one application, the MEMS structure is an accelerometer having a movable component that, in response to an external stimulus, is actuated so as to vary the size of a capacitive air gap. Accordingly, the capacitance output of the MEMS structure provides an indication of the strength of the external stimulus.
One method of fabricating such components, often referred to as surface micro-machining, uses a sacrificial layer such as silicon dioxide that is deposited onto a substrate which is generally single crystal silicon which has been covered with a layer of silicon nitride. A MEMS component material, polycrystalline silicon by way of example, is then deposited onto the sacrificial layer. The silicon layer is then patterned by standard photolithographic techniques and then etched by a suitable reactive ion etching plasma or by wet chemistry to define the MEMS structure and to expose the sacrificial silicon dioxide layer. The sacrificial layer is then etched to release the MEMS component. Such etching and patterning are well known by those having ordinary skill in the art, and are described, for example, in M. Madou, Fundamentals of Microfabrication, (CRC Press, Boca Raton, 1997), or G. T. A. Kovacs, Micromachined Transducers Sourcebook, (WCB McGraw-Hill, Boston, 1998).
It is often desirable to integrate a MEMS structure with an integrated circuit into a single package or onto a single chip. However, many materials are used when fabricating an integrated circuit and during the packaging process, such as water, photoresist, dopants, coatings, etchants, epoxies, etc. The nature of MEMS structures with their inherent mechanical motion is such that the introduction of any of these materials into the structure will most likely render it inoperative. The microscopic mechanical MEMS structure may further be damaged by dirt finding its way into the structure during packaging and handling of the MEMS structure or of the integrated MEMS/circuit pair. Accordingly, a method and apparatus for protecting the MEMS structure from these potential contaminants are desirable.
What is therefore needed is a method for encapsulating a MEMS structure to protect the device from harmful contaminants and other hazards while still allowing an external electrical connection to the device.
The present inventors have recognized that a cap may be bonded to a substrate so as to encapsulate a MEMS structure and provide a seal to protect the device from contaminants and other hazards.
In accordance with a first aspect of the invention, a MEMS structure includes a substrate, at least one conductive element that is in mechanical communication with the substrate and that extends therefrom, a movable MEMS element free from the substrate and positioned such that a gap separates the movable MEMS element from the at least one conductive element, at least one electrical trace having a first terminal end in electrical communication with the at least one conductive element and a second terminal end in electrical communication with a peripheral region, and a cap attached to the substrate inside the peripheral region having upper and side walls that encapsulate the at least one conductive element and the movable MEMS element.
These and other aspects of the invention are not intended to define the scope of the invention for which purpose claims are provided. In the following description, reference is made to the accompanying drawings, which form a part hereof, and in which there is shown by way of illustration, a preferred embodiment of the invention. Such embodiment also does not define the scope of the invention and reference must be made therefore to the claims for this purpose.